1. Technical Field
The present invention relates to a signal generation circuit, a jitter injection circuit, a semiconductor chip, and a test apparatus. More specifically, the present invention relates to a signal generation circuit for generating a signal including jitter injected thereto.
2. Related Art
Devices such as high-speed communication devices and high-speed serial I/O devices are tested for properties, including jitter tolerance testing. For example, it is stipulated to conduct a test by injecting jitter having a frequency of several hundred MHz to communication data according to recommendation of International Telecommunication Union Telecommunication Standardization Sector (ITU-T).
When a high-speed device under test (DUT) is actually used, jitter of a high-frequency component has a significant influence on the bit error. For this reason, it is desired to test the high-speed DUT by using a test apparatus which is capable of injecting jitter having a high frequency.
Here, jitter can be injected into a clock signal, for example, in such a manner that a signal corresponding to the jitter is injected into the control input of a voltage-controlled oscillator or the like which generates the clock signal. In this way, the frequency or phase of the clock signal is modulated, so that the jitter is injected into the clock signal. Also, the jitter can be injected into a data signal by supplying the clock signal including the jitter injected thereto as the driving clock of a pattern generator which generates the data signal.
Here, the jitter is injected into the clock signal by frequency- or phase-modulating the clock signal. Therefore, there is a problem that the frequency of the jitter which can be injected into the clock signal is limited to approximately several dozen MHz.
There is another method for injecting jitter into a signal, in which a variable delay circuit is provided on a transmission path for the signal. According to this method, the jitter can be injected into the transmission signal by controlling the time delay of the variable delay circuit in accordance with the jitter to be injected.
However, it takes a long time to vary the time delay of the variable delay circuit. Therefore, this method also has difficulties in injecting high-frequency jitter.
It may be possible to inject high-frequency jitter by using a high-speed operating variable delay circuit or the like. However, this poses a problem of an increased cost for the circuit.
In view of the above-mentioned problems, an advantage of some embodiments of the present invention is to provide a signal generation circuit, a jitter injection circuit, a semiconductor chip and a test apparatus which can solve the above-mentioned problems. This advantage is achieved by combining the features recited in the independent claims. The dependent claims define further effective specific example of the present invention.